This invention relates in general to image displays including television cathode ray picture tubes, and more particularly is concerned with improvement in image display contrast by a reduction in the reflection of ambient light from the display faceplate.
It has long been a major goal in the image display art to provide a display having a maximum image contrast together with maximum brightness. Brightness has been enhanced, for example, by utilizing more efficient phosphors and, in cathode ray picture tube displays, by impacting the phosphors with electron beams having increased energy.
Image contrast was enhanced in the earlier days of television by means of a neutral density filter (typically about 42% transmissive) positioned over or incorporated in the viewing faceplate. Ambient light striking the faceplate was subject to a first reduction in its passage through the filter toward the phosphor screen, then to a second reduction as it was reflected back to the viewer, again through the filter. A penalty was paid in the use of this system in that light emitted by the phosphor was also attenuated in its single passage through the filter.
A major breakthrough in enhancing image contrast was achieved through the "black surround" system disclosed by Fiore et al in U.S. Pat. No. 3,146,368. A screen structure is disclosed wherein the phosphor deposits, instead of having tangential contact with one another, are reduced in size and separated over the screen area. A light-absorbing pigment is placed in the spaces between these phosphor deposits. The electron beam landing area is larger at least in the horizontal direction then the phosphor deposits. The black surround structure so markedly increases contrast that the faceplate glass can be made clear and the brightness thereby doubled.
Imaging screens having filter particle elements associated with a phosphor element are intended to provide for reduction in ambient light reflection which more than offsets the inevitable loss in brightness. Filters comprise materials that are transmissive to light of certain wavelengths, but absorptive of light of other wavelengths. As indicated by FIG. 1A, part of the beam 1 of white light falling on an optically continuous filter material 2 will be reflected, as indicated by beam 3, while part of the beam will be transmitted through the material as indicated by beam 4. If the filter material is blue, for example, light of all wavelengths other than blue will be absorbed, and only blue light will be able to pass through the filter material 2, as indicated by beam 4. Also, the reflected light indicated by beam 3 will appear to be blue. Examples of optically continuous filter elements usable in picture screens include the lusters manufactured by Englehard Industries of Newark, N.J.
Pigments can be considered as particulate filters with a high index of refraction. With reference to FIG. 1B, a beam 5 of white light is shown as impinging upon a pigment particle 6. The particle is highly absorptive of light of all wavelengths except those in a selected band, or bands. Light of all other wavelengths is quickly absorbed as it enters the particle 6. Light within the selected band or bands is transmitted or reflected internally, exiting the particle 6 in all directions as shown by arrows 7, giving the pigment particle its characteristic color or hue. In effect, the pigment particle "scatters" light falling upon it. If the particle of pigment comprises a blue pigment, for example, only blue light will be scattered, and light of all other wavelengths will be absorbed. The amount of scattering depends, inter alia, upon the size of the particle, with maximum scattering occurring when the particle is of about the same size as the wavelength of the light impinging upon it. Examples of pigments include titanium dioxide, a white pigment; cobalt, a blue pigment; and cadmium sulfoselenide, a red pigment.
Kaplan in U.S. Pat. No. 2,959,483 discloses a color image reproducer and method of manufacture. One embodiment (see FIG. 2 herein) comprises green, red and blue target elements 10, 12 and 14 which are located between faceplate 16 and aluminum film 18. Each target element comprises two discrete layers: a phosphor layer 20 and a filter layer 22. Layer 20 comprises a specific color phosphor for each target element; for example green phosphor 24 in target element 10. Filter layer 26 in target element 10 comprises a continuous green filter material which selectively transmits light of the wavelength corresponding to the green primary color while sharply attenuating the red and blue primaries. The effect of color filter layer 26 upon three impinging rays of ambient light is illustrated by lines 28, 30 and 32. Line 32 represents light of a wavelength corresponding to green in color, while lines 28 and 30 represent red and blue, respectively. Because of the selective light transmission characteristics of the color filter layer 26, the red and blue light represented by rays 28 and 30 is sharply attenuated in passing through filter material 26 as light of such colors is reflected from the target element. In fact, filter 26 attenuates the impinging light twice, once each time it is required to traverse the filter material. The same effect takes place with regard to the red target element 12 and the blue target element 14. The result is that much if not all of the ambient light impinging upon the target structure is absorbed by the respective filter layers to improve image contrast and color saturation under high-level ambient viewing conditions, and without substantially reducing image brightness.
This approach is feasible because an optically continuous (non-particulate) filter layer will transmit a major fraction of the light having wavelengths within its bandpass. Optically continuous filter materials, as presently known however, are impractical since their deposition methods are too costly for use in mass-production. Because of the expense entailed in their use, optically continuous filters, so far as is known, have never been used commercially in the manufacture of color cathode ray tubes.
Another embodiment of Kaplan '483 teaches the use of mixtures of luminescent materials and pigment-type color filter material having selective color-transmissive characteristics corresponding to the emission characteristics of the associated phosphor. A further embodiment of the '483 disclosure is similar to the discrete phosphor-filter layer system of FIG. 2, except that the luminescent layer comprises a homogenous mixture which emits light in all three of the primary colors selected for image reproduction; i.e. white light.
U.S. Pat. No. 3,886,394 to Lipp discloses an image display employing phosphor particles which are filter-coated. FIG. 3 indicates, according to Lipp, a light-emitting phosphor particle 33, the surface of which is coated with filter particles 33A comprising a pigment which is said to absorb spectral components of light from ambient sources. It is alleged that by only partially covering, in the range of 20 to 80%, the phosphor particles with filter particles, the transmission, absorption and reflection of light may be "tailored" to optimize the brightness and contrast of the display image where the ambient light level is relatively high. Two embodiments are disclosed: one in which there is a single layer of phosphor particles coated as described, and the other consisting of two layers comprising the coated phosphor particles, and over this, a layer of phosphor particles which are uncoated.
Uehara et al, in an article entitled "High Contrast Color Picture Tube" (Hitachi Review, Vol. 27 (1978), No. 4) reviews various filter phosphor screening techniques as follows (quoting directly from the article):
"Phosphor screening processes with pigments are generally classified into the following three kinds depending upon how the pigment is involved in the screen:
(1) Filter preparation process PA1 (2) Slurry mixture process PA1 (3) Pigmented phosphor process PA1 Pat. No. 426,789--Siemens (German) PA1 U.S. Pat. No. 2,644,854--Sziklai PA1 U.S. Pat. No. 2,750,525--Palmer PA1 U.S. Pat. No. 2,848,233--Yanagisawa et al PA1 U.S. Pat. No. 2,913,352--Windsor PA1 U.S. Pat. No. 3,013,114--Bridges PA1 U.S. Pat. No. 3,454,715--Larach et al PA1 U.S. Pat. No. 3,812,394--Kaplan PA1 U.S. Pat. No. 4,087,280--Stookey et al
"In the filter preparation process, the pigment layer is placed between a glass panel and the ordinary phosphor screen so that it works as a cut-off filter against any colors other than those emitted from the phosphor used. This conception has been known widely since before the black matrix screen was introduced. However the complexity of the process has prevented it from being applied to practical production.
"The slurry mixture process is far easier in the use of pigments, because they are simply mixed into the ordinary phosphor slurry which is very common for screening. In this case, however, it is very difficult to completely avoid so-called pigment cross contamination which usually results in a brightness loss in the finished tube.
"After carefully studying these processes, we concluded that they could not be used for our purpose and a third process should be sought.
"Finally we developed the pigment phosphor process. Blue phosphor was coated with the blue pigment cobalt aluminate, red phosphor with the red pigment ferric oxide."
In an article entitled "Black Stripe High Contrast Color Picture Tube," by Ikegaki et al. (Toshiba Review, August 1976), a "graduated" pigment system is disclosed in which the blue phosphor particles have a pigment coating and in which the concentration of pigment varies through the blue phosphor field, with the heaviest concentration of pigment being nearest the screen. A large increase in contrast over the standard Toshiba black-stripe tube is alleged.
The mixing of filter materials or pigment particles with the phosphor can result in undesired side effects such as cross-contamination. In addition, such mixing can reduce the brightness of the images produced by the associated phosphor particles by shielding the phosphor particles from energizing electrons, and by absorption of the light emitted by the phosphor particles near the point of origin. As will be explained in more detail hereinafter, a pigmented phosphor material can be used to produce a very slightly improved picture if the amount of pigmentation is very modest--e.g., that amount which reduces ambient light reflection, in comparison to one that is not pigmented, in the approximate range of 15% to 20%. Attempts to make a "deep filter" tube or screen using the prior art pigmented phosphor approach resulted in excessive brightness losses. In the context of this application, a "deep filter" screen is considered to be so filtered that a reflectivity reduction of about 35% to 50% is provided with a corresponding higher picture contrast.
Hoyt in U.S. Pat. No. 2,828,435 discloses means for making television screens in cathode ray tubes by a decalcomania process. In one of many illustrative decalcomania embodiments, a backing sheet is surmounted by pigment layer, a phosphor layer, and a protective clear surface layer. The backing sheet is removed, and the assembled layers are applied to the faceplate to form the screen with the pigment layer lying closest to the faceplate. The stated purpose is (quoting) " . . . to slightly shade or color the image of cathode ray tubes."
In U.S. Pat. No. 2,858,234, Ishler discloses a method for coating the inside wall of a fluorescent lamp envelope with a color-subtractive pigment said to have a uniform thickness and freedom from streaking. The pigment, which operates by subtraction or absorption of undesired color to increase the proportion of the desired color in the spectral output is said to achieve a greater intensity or saturation of the desired color of the light emitted by fluorescent illuminating lamps.
Barnes U.S. Pat. No. --2,599,739 discloses the provision of a reflection-reduction coating between the material of the inner face of a cathode ray tube and the fluorescent screen. The coating is alleged to reduce halation by eliminating to a substantial degree the amount of light reflected from the surfaces of the tube face onto the fluorescent screen. The preferred coating is said to be formed of "submicroscopic, microgranular" (sic) discrete approximately spherical particles less than 625 angstroms in diameter which are deposited on the glass surface so as to form minute projecting irregularities on the surface. Further, the concentration of the particles in the irregularities is said to decrease from the surface of the tube face outward, forming angularities which are alleged to increase the transmission of light rays from the surface with a consequent reduction in reflected light rays.
In U.S. Pat. No. 3,614,503 to Dietch, assigned to the assignee of the present invention, a screen is disclosed for a color cathode ray tube comprised of interleaved deposits of phosphor material which emit light of different colors; the deposits are surrounded by a light-absorbing material. A plurality of diffusely reflecting materials is superposed over the phosphor deposits and the light-absorbing material. The reflecting material and the light-absorbing material are spaced with respect to one another to simulate a multiplicity of integrating spheres surrounding the phosphor deposits. It is said that multiple reflections from the surfaces permit light developed by the phosphor dots and otherwise attenuated in the light-absorbing material to be added to the useful light output of the tube.
Dietch in U.S. Pat. No. 3,952,225, assigned to the assignee of the present invention, discloses a cathode ray tube image screen wherein a substantially non-reflective grille is provided on the faceplate. A reflective grille is provided in registration with the non-reflective grille, with a phosphor screenover the reflective grille backed by an aluminum film. Light emitted in the open phosphor areas find its way out directly through the faceplate. However, light emitted by the excited phosphor areas behind the black grille (as seen from the viewing side), instead of being dissipated by the light absorption of the black grille, is subjected to multiple reflections between the reflective grille elements and the aluminum backing layer. This light is said to eventually reach the open phosphor areas whereupon it is also emitted by the faceplate. The invention is said to provide a new and greatly improved high-brightness, high-contrast cathode ray tube image screen for use in either monochrome or color picture tube environments.